Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices.
In general, image scanners use an optical lens system to focus an image onto an array of photosensors. Photosensor arrays typically have thousands of individual photosensitive elements. Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document defining a picture element (pixel) on the image being scanned. Optical sampling rate is often expressed as pixels per inch (or mm) as measured on the image being scanned. For opaque images, for example, photographs or printed pages, a typical scanner optical sampling rate is 600 pixels per inch (24 pixels per mm).
One important specification for scanning is the overall time required to scan an image. One important factor is the data transfer rate between the scanner and a host computer. High sampling rates generate large quantities of data, and at high sampling rates the data transfer rate may limit overall scanning time. Another important factor that can affect the overall time required to scan an image is the sensor exposure time. Typically, in a photosensor circuit, current generated by light is used to change the charge on a capacitor. A finite time is required to change the charge on a capacitor sufficiently to ensure an acceptable signal to noise ratio. At lower sampling rates, sensor exposure time may limit the overall time required to scan an image. The required sensor exposure time may be reduced by increasing the intensity of illumination on the document, or by increasing the aperture size of the optics system (thereby capturing more of the light reflected from the document). Either approach adds cost. In particular, the cost of a lens system increases substantially with aperture size, partly because controlling lens aberrations becomes more difficult and expensive as the lens aperture size increases. There is a need for reducing exposure time in a digital image scanner without substantially increasing cost.